How to control that project attachments cannot be deleted

Do you know how to control that project attachments cannot be deleted?

My client wants to attach an important contract document to the project so that it cannot be deleted after the project is activated.

Or if you have information on how to do this by using the Document function of the Project Contract instead of the Project, it would be helpful if you could share it with us.

If you have any good ideas, please let me know.

Thanks,

Mayuko